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NSC 2201 (16)
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Chapter 5

NSC 2201 Chapter 5: CH 5 Synaptic Transmission

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NSC 2201

CH 5 SYNAPTIC TRANSMISSION p. 109-142 INTRO • Process of info transfer at a synapse is synaptic transmission • There are electrical synapses and chemical synapses TYPES OF SYNAPSES ELECTRICAL SYNAPSES • Take place in specialized sites called gap junctions • Allow many non-neural cells to communicate • Connexons allow ions to pass directly from cytoplasm of one cell to cytoplasm of the other • Unlike vast majority of chemical synapses, gap junctions are bidirectional • Transmission at electrical synapses is fast and reliable, used in escape reflexes • Postsynaptic potential (PSP) formed in postsynaptic since small amount of current flows from presynaptic neuron • Often found where normal function requires neighboring neurons to be highly synchronized • May help to control fine timing of motor control • During prenatal and postnatal brain development, gap junctions allow neighboring cells to share electrical and chemical signals to coordinate their growth and maturation CHEMICAL SYNAPSES • Most synapses in mature brain are chemical • Secretory granules AKA dense-core vesicles contain soluble protein that appears dark in electron microscope • Has synaptic vesicles that hold NT • Membrane differentiations: dense accumulations of protein adjacent to and within membranes on either side of synaptic cleft • Active zones: sites of NT release • Postsynaptic density: protein thickly accumulated in and just under postsynaptic membrane, has receptors that turn intercellular change in membrane potential into intercellular signal CNS Chemical Synapses • Axodendritic: postsynaptic membrane is on dendrite • Axosomatic: postsynaptic membrane on cell body • Axoaxonic: postsynaptic membrane on axon • Axospinous: presynaptic axon contacts postsynaptic dendritic spine • Dendrodenritic: dendrites form synapses with each other • Asymmetrical synapses (Gray’s type I synapse): membrane differentiation on postsynaptic side is thicker than that on presynaptic side, usually excitatory • Symmetrical synapses (Gray’s type II synapse): membrane differentiation are of similar thickness on both sides, usually inhibitory The Neuromuscular Junction • Neuromuscular junction: chemical synapses between axons of motor neurons of spinal cord and skeletal muscle • Fast and reliable • One of largest synapses in body • Presynaptic terminal has large number of active zones • Motor endplate AKA postsynaptic membrane: has series of shallow folds • Presynaptic active zones are aligned with shallow folds and postsynaptic membrane of folds is packed with NT receptors PRINCIPLES OF CHEMICAL SYNAPTIC TRANSMISSION NEURTRANSMITTERS • Fall into three categories: o 1. Amino acid o 2. Amines o 3. Peptides NEUROTRANSMITTER SYNTHESIS AND STORAGE • Depolarization of terminal membrane > voltage-gated calcium channels in active zones open > influx of Ca 2+ > Ca 2+floods cytoplasm of axon terminal > elevation in [Ca ] is signal that causes NT to be released from synaptic vesicles • Exocytosis by vesicles to release NT • Vesicle membrane later recovered by endocytosis • Periods of prolonged stimulation can mobilize vesicles from reserve pool bound 2+ to cytoskeleton of axon terminal, triggered by elevations of [Ca ] i • Release of peptide NT is different o Physically further away from [Ca ] so needs high-frequency trains of APs i so [Ca ] tiroughout terminal can build up o One level reached to trigger secretory granule release, it’s quite slow, 50 msec NEUROTRANSMITTER RELEASE NEUROTRANSMITTER RECEPTORS AND EFFECTORS Transmitter-Gated Ion Channels • Transmitter-gated ion channels open and allow ions to flow when NT binds to specific site on extracellular region of channel • Not as selective as voltage-gated channels • EPSP: excitatory postsynaptic potential • IPSP: inhibitory postsynaptic potential G-Protein-Coupled Receptors • G-protein-coupled receptors has slower, longer lasting effects than amino acid or amine NT • NT molecules bind to receptor proteins > receptor proteins activate small G- proteins > G-proteins free to move along intracellular face of postsynaptic membrane > activated G-protein activate “effector” proteins • Effector proteins can be G-protein-gated ion channel or they can be enzymes that produce second messengers that diffuse away in cytosol • Second messengers > activate addi
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